1. Field of the Invention
The present invention relates to an aggregate of carbon nanotubes. Further specifically, it relates to an aggregate of carbon nanotubes and a dispersion thereof, a transparent conductive film, and a field emission material.
2. Description of the Related Art
In 1991, carbon nanotubes were widely reported for the first time. A carbon nanotube has a tubular shape that substantially one face of graphite is wound, one wound in one layer is called a single-walled carbon nanotube, and one wound in multilayer is called a multi-walled carbon nanotube. Among multi-walled carbon nanotubes, in particular, one wound in two layers is called a double-walled carbon nanotube. Carbon nanotubes are expected to be used as a conductive material having excellent intrinsic electrical conductivity by themselves.
As a method for producing carbon nanotubes, an arc discharge method, laser evaporation method, chemical vapor deposition method and the like are known. Of the chemical vapor deposition methods, there is known a catalyst chemical vapor deposition method which is conducted by supporting a catalyst on a support.
Among carbon nanotubes, it is known that a single-walled carbon nanotube is high in characteristics such as electrical conductivity and heat conductivity because it has a high graphite structure. However, since the single-walled carbon nanotube has a strong and very thick bundle structure, it cannot exhibit a nano-effect that each carbon nanotube possesses, and it has been difficult to develop various applications thereof. In particular, since the dispersion in a resin or solvent is difficult, its expected high characteristic cannot be exhibited, and the present situation is that the development of various applications has been hindered. In particular, it has been difficult to exhibit a practical performance in applications as a transparent conductive film, molded article, membrane or the like using the carbon nanotube
Of multi-walled carbon nanotubes, carbon nanotubes with relatively a few number of layers, 2 to 5 layers, have both characteristics of those of single-walled carbon nanotube and multi-walled carbon nanotube, thus there have been drawn attentions as a promising material in various applications. Above all, it is thought that a double-walled carbon nanotube has the most excellent characteristic, and several synthesis methods have been developed. Recently, as a synthesis method of double-walled carbon nanotubes with high purity, a method of Endo et al. is known (Japanese Unexamined Patent Publication No. 2005-343726; Nature, vol. 433, 476 (2005), Chemical Physics Letters, 414 (2005) 444-448; Journal of American Chemical Society, 128 (2006) 12636-12637). In this method, a carbon source is reacted by disposing an iron salt as a main catalyst and molybdate as a co-catalyst to synthesize a double-walled carbon nanotube. Further, as an application for the double-walled carbon nanotube thus obtained, since a double-walled carbon nanotube has high heat stability, an application as a field emitter used in a high electric current is described.
However, a double-walled carbon nanotube with high quality forms, in the same manner as a single-walled carbon nanotube, a strong bundle through a hydrophobic interaction between tubes and interaction between pi-electrons, and it has been thought to be difficult to disperse the carbon nanotube. It is thought that a double-walled carbon nanotube according to Endo et al. forms similarly a strong and thick bundle. As the indirect evidence for having a strong and thick bundle structure, heat stability of aggregate of carbon nanotubes is mentioned. It is assumed that an aggregate of carbon nanotubes with high heat stability forms a thicker bundle structure (Non-patent document 3). The heat stability of carbon nanotubes can be determined by combustion peak temperature in air. Combustion in air is thought to be an oxidation reaction by attack of oxygen molecules. Even if each is the same carbon nanotube, a bundle thereof is thick, namely, in a bundle that more carbon nanotubes get together, inner carbon nanotubes hardly undergo the attack of oxygen, thus oxidation reaction hardly occurs, and the combustion peak temperature of aggregate of carbon nanotubes rises. Reversely, when a bundle is thin, namely, in a bundle that a few carbon nanotubes get together, it is thought that inner carbon nanotubes also easily undergo the attack of oxygen, thus the combustion peak temperature of aggregate of carbon nanotubes is lowered.
Carbon nanotubes described in the foregoing documents are those produced in the same synthesis method, as described in Journal of American Chemical Society, the combustion peak temperature is as high as 717° C., it is thought that these carbon nanotubes form a strong and thick bundle, and they were not sufficient in the case where dispersibility of high level was required.
On the other hand, regarding multi-walled carbon nanotubes with more numbers of layers than those described above, generally the diameter is large, there are many defects in the graphite layer, because it is more difficult to form a bundle than the above-described carbon nanotubes with a few number of layers, dispersibility is superior. However, since such multi-walled carbon nanotubes are inferior in quality, it has been difficult to exhibit practical performance in applications as a transparent conductive film, molded article, membrane or the like requiring particularly excellent light transmittance and surface resistance.